This invention relates to apparatus for forming images for heads-up displays and, more particularly, to apparatus for collimating a projected image at or near infinity.
Helmet mounted displays (HMD's) having many different types of optical collimating systems are known in the art. These optical systems generate an image appearing to be located very far from the observer from a small image source magnified by its placement very near to the observer's eye.
Berman & Melzer, U.S. Pat. No. 4,859,031 (1989), teaches a vision immersion modules.RTM. (VIM.RTM.) type of optical collimating system. FIG. 1 illustrates a Berman & Melzer apparatus. In FIG. 1, the apparatus 100 includes an image source 110, a lens 120 and a cholesteric liquid crystal (CLC) 130. An observer viewing the image produced by the apparatus 100 is represented by an eye E. (FIG. 1 is not drawn to scale.)
The lens 120 is a meniscus, coated on its concave side to reflect light. The lens 120 is, therefore, a partially reflective meniscus.
Berman & Melzer utilizes unique properties of cholesteric material. A CLC device will reflect those components of light that are within a particular bandwidth B, and of a particular rotary sense of circular polarization. Light reflected by a CLC will keep its original rotary sense. Thus, for example, light projected upon a CLC device constructed to reflect light components within a bandwidth B.sub.1 and having a right hand circular polarization (RHCP) will pass all other components of that light, including light components within the bandwidth B.sub.1 and having a left hand circular polarization (LHCP). The reflected light maintains its original polarization (RHCP).
The image source 110, lens 120 and CLC 130 are aligned orthgonally to and optically along an axis A to operate as described below. LHCP light from the image plane 110 partially passes through the meniscus lens 120 and strikes the CLC 130. The CLC reflects the LHCP light back to the meniscus lens 120, without changing its polarization handedness. The reflectively coated side of the lens 120 in turn reflects the LHCP light back to the CLC 130, changing the polarization of the light to RHCP. The RHCP light can now pass unobstructed through the CLC and out to the eye E.
Although the Berman & Melzer apparatus 100 is both compact and lightweight, the efficiency of the CLC 130 of the VIM.RTM. optical collimating assembly 100 is not 100%. Some light within the bandwidth B.sub.1 and having the correct polarization handedness nonetheless leaks through the CLC 130 instead of being reflected back towards the meniscus lens 120. This light passes directly to the eye and forms an unwanted image called a "ghost image". With the VIM.RTM. optical assembly of FIG. 1, the ghost image lies directly in the center of the desired image.
Berman, U.S. Pat. No. 5,050,966 (1991), teaches an optical collimating apparatus. FIG. 4 illustrates a Berman combiner apparatus 400, in which a combiner element 410 transmits all components of light from image sources 420 (through a collimating optic 430) and 460 out of the line of sight of an observer E, except for light of the desired polarization handedness and bandwidth. The Berman combiner element 410 reflects this desired light at an angle into the line of sight, through a CLC 440 and semi-reflective mirror 450, toward the observer E.
The combiner 410 of the Berman apparatus, however, increases the complexity and size of the apparatus 400. Further, the combiner 400 forces an angle into the apparatus 400 which further affects the size requirements of any display system incorporating it.
Also, the Berman apparatus 400 assumes that its CLC 440 is 100% efficient. While such an assumption is reasonable for modeling the problem which Berman addresses, in actuality the CLC 440 leaks, with the attendant problems described above. Finally, in those configurations where Berman does not include a combiner 410, there is no angle with which to differentially reflect desirable and undesirable light.
Trissel and DeFoe, U.S. Pat. No. 5,408,346 (1995), teaches an optical collimating apparatus for focusing an image. (Trissel and DeFoe is assigned as well to the assignee of the instant invention.) FIG. 5A illustrates the Trissel and DeFoe apparatus 500, including a CLC 510 and a mirror 520. The CLC 510 reflects light within a predetermined bandwidth and having a predetermined circular polarization while transmitting all other light. The mirror 520 expands (collimates) light received.
In the apparatus 500, the CLC 510 is positioned to transmit image light from an image source 530 to the concave mirror 520, reflecting light having the predetermined frequency and handedness characteristics out of the line of sight of an observer E. The mirror 520 expands (collimates) the image light received from the CLC 510 and reflects the collimated light back to the CLC 510, reversing its handedness. The CLC transmits this light to the line of sight of the observer.
FIG. 5B shows an alternative embodiment of the Trissel and DeFoe apparatus.
Accordingly, there is a need for an optical collimating system that generates an image without producing ghost images in the field of view of the generated image, using as simple a configuration and as few components as possible.
These and other objects of the invention will be readily apparent to one of ordinary skill in that art on the reading of the background above and the description herein.